This invention relates to the pyrolysis of solids containing organic material and is particularly concerned with a process and apparatus for retorting oil shale or coal in which the heat required for the retorting process is supplied indirectly to the retorting zone by burning the residual organic material left in the retorted shale or coal.
Because of a dwindling supply of petroleum liquids from underground reservoirs, attention has recently been focused on the recovery of hydrocarbon liquids and gases from solids such as oil shale, coal, industrial and municipal solid wastes and the like. Work by both governmental agencies and private industry has demonstrated that the organic material in such solids can be converted with varying degrees of difficulty into volatile hydrocarbonaceous fluids such as combustible gases, motor fuels, heating and fuel oils, and various by-products which have value in chemical and petrochemical industries. In general, the more attractive of the recovery techniques previously proposed involve the heat treatment of such solids in a manner sufficient to distill or otherwise decompose the organic material into the above-mentioned volatile hydrocarbonaceous products. Such techniques, which are commonly referred to as retorting or pyrolysis processes, take on a multitude of forms, including batch or continuous schemes utilizing fixed, moving or fluidized beds wherein either a portion of the solid organic material itself is combusted to supply the pyrolysis heat, or the pyrolysis heat is generated externally and supplied to the process via a gaseous, liquid or solid heat carrier.
Oil shale is considered to be one of the best candidates of all carbon-containing materials for processing in such a retorting or pyrolysis scheme since it comprises a mixture of a minor amount of solid organic matter called kerogen and a major amount of mineral matter. Because of the physical and chemical compositions of oil shale, its organic content has not been found to be economically recoverable by any technique other than the application of heat via pyrolysis or retorting. When mined oil shale is retorted, the solid organic matter undergoes destructive pyrolysis and a large percentage of the organic matter is converted to liquid and light gaseous hydrocarbonaceous products with the remainder staying as a carbon-rich residue in the mineral matrix. Processes for recovering hydrocarbonaceous products from raw oil shale are generally classified into four categories according to the method in which the heat is supplied. These categories are as follows: (1) heat is transferred from an external source through the walls of the retorting vessel; (2) heat is supplied by direct combustion in the retorting vessel; (3) heat is supplied by passing an externally heated gas into the retorting vessel; or (4) heat is supplied by introducing externally heated solids into the retorting apparatus. The known processes which fall in category (1) have a major disadvantage in that there is a substantial amount of organic matter left in the retorted oil shale and this in turn substantially decreases the economics of the overall process. The processes encompassed by category (2) in which combustion is carried out in the retort itself avoids the problem of not converting a substantial amount of the organic material present but have disadvantages in that the product is diluted by the combustion gases produced, the naphtha yields are lower because the light vapors burn and there is a tendency for the shale to overheat and form clinkers in the retort.
The indirectly heated retorting processes of categories (3) and (4) while having advantages over the processes in categories (1) and (2) still have major disadvantages. The use of an externally heated gas to supply heat enables the gas and solids residence times to be independently controlled and may result in greater than Fischer Assay yields. Unfortunately, such processes require high gas rates, result in a product diluted by the externally added gas, leave unconverted organic material in the retorted shale, and pose a high potential for solids carryover. The processes of category (4) in which the raw shale is contacted with externally heated solids, preferably produced by combusting spent shale produced in the retort, avoid dilution of the product gas but have disadvantages which include the requirement for additional expense of solids handling apparatuses and procedures, and a decrease in product yield caused by the adsorption of pyrolysis products on the heat carrier solids. Some of the processes which fall in categories (3) and (4) often yield a spent shale which contains as much as 30 percent of original feed carbon. This results in low thermal efficiency and poses potential waste disposal problems.